Specify "text/html" for file:// scheme URL for Gecko mimetype detector
Specify "text/html" for file:// scheme URL for Gecko mimetype detector

Gecko mimetype detector always fallback application/xml when
Content-Type header is empty.
But vivliostyle.js depends on DOMParser behavior for text/html file.
In Gecko, local file will treat as application/xml even if specifing
Content-Type header.
So, we should override mimetype for file:// scheme URI which terminates
htm or html.

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js